The present invention relates generally to intravascular ultrasound imaging (IVUS). More particularly, the invention describes a novel IVUS system with high contrast resolution and high spatial resolution using pulse compression and frequency compounding.
IVUS imaging is generally performed for assessment of coronary artery disease and to guide percutaneous coronary interventions, typically the placement of a stent.
Atherosclerotic lesions that are prone to rupture, so called vulnerable plaques, are of increasing interest to interventional cardiologists. One type of vulnerable plaque thought to be responsible for a large percentage of plaque ruptures is a thin-cap fibroatheroma wherein a thin (<65 μm) fibrous cap overlies a mechanically unstable lipid-rich or necrotic core. Current commercially available IVUS systems operate up to only 40 MHz and have axial resolutions that are limited to approximately 100 μm. Consequently, current commercially available IVUS systems cannot measure the cap thickness of vulnerable plaques.
Resolution of current commercial IVUS is inadequate to characterize neointima, the thin layer of tissue that forms over a stent as the vessel heals post-deployment. Stent struts could be better visualized and their apposition assessed with higher resolution imaging. Other features of pathological interest such as thrombus, small dissections, and small calcifications can be better visualized with higher resolutions imaging.
It is generally necessary to increase the transducer frequency in order to improve spatial resolution of the IVUS system. However, increased imaging frequency also leads to reduced contrast between blood and non-blood tissue that in turn makes segmentation of the blood-filled lumen from the intimal plaque difficult. Increasing transducer frequency may also suffer from higher tissue attenuation leading to lower Signal-to-Noise Ratio and resulting in lower contrast resolution. Some automatic segmentation algorithms exploit the frequency-dependent ultrasound properties of blood and non-blood tissues as described for example in U.S. Pat. No. 5,876,343 by Teo. Real-time, automatic segmentation tools are often prone to errors, which reduce their utility in clinical practice.
Some prior art examples of multi-frequency imaging systems are disclosed in U.S. Pat. Nos. 5,876,343 and 6,139,501 and U.S. Patent Application Publication No. 2011/0087104, which are incorporated herein by reference in their respective entireties.
Multi-frequency IVUS imaging can also be generally achieved by use of multiple transducer imaging catheters. However, multiple transducers add complexity and cost to the disposable imaging catheter and the imaging system. The potential need to co-register the images from the separate transducers further complicates their practical use.
There exists a need for a technology that provides higher contrast resolution for improved assessment of coronary arteries while still providing sufficient spatial resolution to characterize stent healing and vulnerable plaques. Further, it is desirable that such a technology does not require any co-registration step between multiple images. Still further, it is desirable that such a technology does not substantially increase system and catheter complexity and cost over existing commercial systems and catheters.